---
id: 656879a66338b2a461d5d307
title: ステップ 9
challengeType: 20
dashedName: step-9
---

# --description--

<!-- TODO: Find better places to split explanation up. -->

The Luhn algorithm is as follows:

1. From the right to left, double the value of every second digit; if the product is greater than 9, sum the digits of the products.
2. Take the sum of all the digits.
3. If the sum of all the digits is a multiple of 10, then the number is valid; else it is not valid.

Assume an example of an account number "7992739871" that will have a check digit added, making it of the form 7992739871x:

```markdown
Account number      7   9  9  2  7  3  9   8  7  1  x
Double every other  7  18  9  4  7  6  9  16  7  2  x
Sum 2-char digits   7   9  9  4  7  6  9   7  7  2  x
```

Replace the `pass` statement with a variable `sum_of_odd_digits` and a value of `0`.

# --hints--

You should have `sum_of_odd_digits = 0` within the `verify_card_number` function.

```js
({
    test: () => {
        const transformedCode = code.replace(/\r/g, "");
        const verify_card_number = __helpers.python.getDef("\n" + transformedCode, "verify_card_number");
        const { function_body } = verify_card_number;

        assert.match(function_body, /sum_of_odd_digits\s*=\s*0/);
    }
})
```

You should not have `pass` within the `verify_card_number` function.

```js
({
    test: () => {
        const transformedCode = code.replace(/\r/g, "");
        const verify_card_number = __helpers.python.getDef("\n" + transformedCode, "verify_card_number");
        const { function_body } = verify_card_number;

        assert.notMatch(function_body, /pass/);
    }
})
```

# --seed--

## --seed-contents--

```py
--fcc-editable-region--
def verify_card_number(card_number):
    pass

--fcc-editable-region--
def main():
    card_number = '4111-1111-4555-1142'
    card_translation = str.maketrans({'-': '', ' ': ''})
    translated_card_number = card_number.translate(card_translation)

    print(translated_card_number)

    verify_card_number(translated_card_number)

main()
```
